1. Field of Invention
The present invention generally pertains to electron beam lasers and is more specifically directed to bonding a semiconductor crystal to an electrically insulating, thermally conductive stratum for use in such lasers.
2. Description of the Prior Art
Electron beam lasers are described in U.S. Pat. Nos. 3,558,956, 3,602,838 and 3,757,250. A preferred electron beam laser includes a sealed evacuated tube; an electron gun within the tube for providing an electron beam; a light resonant cavity including a direct band-gap semiconductor crystal having a pair of broad optically smooth opposing major surfaces, and means for providing two almost totally reflective surfaces positioned for defining the cavity, with one reflective surface being more reflective than the other; and an electrically insulating, thermally conductive stratum bonded to one of the broad surfaces of the crystal for dissipating heat from the crystal. The cavity is positioned so that the crystal may be bombarded by the electron beam and thereby excited into stimulated emission for emitting coherent electromagnetic radiation from the crystal through the least reflective surface.
An electron beam laser may further include a deflection means for scanning the electron beam, wherein when one of the major broad crystal surfaces is scanned by the electron beam, coherent electromagnetic radiation is emitted in a pattern corresponding to said electron beam scan.
Sapphire is preferred as a stratum material because it is transparent to the produced electromagnetic radiation and because of its ability to dissipate from the crystal, the heat produced upon excitation, without distorting its shape or that of the crystal. Quartz, diamond or BeO may be used in lieu of sapphire.
Direct band gap semiconductors which may be used as the laser crystal include CdS, CdSe, CdS.sub.x Se.sub.(1-x), ZnO and GaAs.
Silver is a commonly used reflective coating.
In some embodiments, the means for providing the reflective surfaces includes a reflective coating on at least one of the broad surfaces of the crystal, and the stratum is bonded to a reflectively coated crystal surface. In other embodiments the light resonant cavity includes the stratum, and the stratum is bonded to an uncoated crystl surface.
Certain difficulties have been encountered with various means which have been employed to bond the crystal to the stratum. One such bonding means is an adhesive layer uniformly applied between the crystal and the stratum. However, most adhesives which have been so applied do not form a satisfactory bond. Particularly unsatisfactory is the bond produced by a solvent based adhesive, since upon forming a very thin adhesive layer between crystal and stratum, the solvent becomes trapped in this layer, and forms gas products which are not conducive to good optical quality or good thermal conductivity. One adhesive which does provide an otherwise satisfactory bond, phenyl ether dicyanate resin, has such a high curing temperature, however, that the properties of the crystals are so affected during the curing of the adhesive that lack of uniform lasing over the breadth of the crystal surface may result after curing.
An adhesive free bonding process described in U.S. Pat. Nos. 3,397,278 and 3,417,459 also has been found unsatisfactory for bonding a crystal to a stratum for use in an electron beam laser. Attempts to bond direct band-gap semiconductor crystals to sapphire have not been successful. In order to achieve a bond between such semiconductor crystals and quartz by such a process it has been necessary to use such high temperatures that uniform laser emission over the breadth of the crystal surface could not be achieved after such bonding.